Improve the Snowplow guide/docs introduction

What does this MR do?

Related to #328665 (closed)

The introduction to our Snowplow docs was outdated. This MR removes a section talking about the then-future plans about standardizing our Snowplow Schema (Closed at &5218 (comment 573797452)), and updates old links to the Snowplow page.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.